1. Field of the Invention
The present invention relates to an image display apparatus and a control method thereof.
2. Description of the Related Art
Recently liquid crystal display apparatuses, having a light emitting diode (LED) backlight as a light source, are becoming popular. Particularly the use of liquid crystal display apparatuses having a direct backlight where a plurality of LEDs are arrayed are rapidly spreading. In the case of a direct backlight, local dimming control for partially adjusting the amount of luminescence can be performed relatively easy. By performing local dimming, the contrast ratio (ratio of the maximum brightness and the minimum brightness) can be improved, and power consumption can be decreased.
Available methods of adjusting the amount of luminescence of a backlight are a method of adjusting a value of current to be supplied to LEDs, and a method of adjusting the lighting duration of LEDs. In the case of the method of adjusting the current value to be supplied to LEDs, emission brightness (instantaneous value) of the LEDs can be decreased and the amount of luminescence of the LEDs can be decreased by decreasing the current value. In other words, the brightness of the backlight can be decreased by decreasing the current value. And the emission brightness of the LEDs can be increased and the amount of luminescence of the LEDs can be increased by increasing the current value. The brightness of the backlight can be increased by increasing the current value. In the case of adjusting the lighting duration of LEDs, the amount of luminescence of the LEDs can be increased by extending the lighting duration, and can be decreased by shortening the lighting duration.
Generally the method of adjusting the lighting duration is used because power efficiency is high. Adjustment of the lighting duration is referred to as pulse width modulation (PWM) control. In PWM control, the ratio (duty ratio) in a predetermined duration (emission control duration) is changed.
LEDs have characteristics where individual difference, time related deterioration and temperature deterioration are considerable. Hence in the case of the above mentioned direct backlight, it has been difficult to keep the amount of luminescence of the backlight (e.g. amount of luminescence in the emission control duration) constant and uniform over a long duration (a long period). Therefore as a method for keeping the amount of luminescence constant and uniform, a method of detecting the emission brightness of each light source unit using an optical sensor installed near the backlight and controlling the amount of luminescence by feedback has been proposed (e.g. see Japanese Patent Application Laid-Open No. 2006-40764). In the feedback control, the current value to be supplied to each light source unit is corrected so that the detected value becomes close to the target value, for example. A light source unit is a minimum unit of the backlight of which the amount of luminescence can be controlled, for example. In other words, the backlight has a configuration that allows the amount of luminescence to be controlled for each light source unit.
However noise normally enters the detection result of emission brightness (e.g. voltage value corresponding to the emission brightness). This makes it necessary to increase the accuracy of the detection result by acquiring the detection value of the emission brightness a plurality of times, or blocking high frequency noise using an LPF (Low Pass Filter) installed on a sensor circuit. Performing such processing increases the time length required for the optical sensor to detect the emission brightness. Therefore if the single lighting duration is short (e.g. lighting duration is set to be short to decrease the amount of luminescence of the backlight when the display mode of the image display apparatus is in low brightness mode, in which brightness on the screen is darkened). If detection of the emission brightness cannot be completed within the lighting duration, the detection result may not be acquired, or an erred detection result may be acquired, which makes it impossible to control the amount of luminescence of the backlight (light emitting unit) at high precision.